A laser device generally produces a beam of coherent light that has a wavefront of relatively small cross-section. In spite of the small cross-section and the coherency of the beam, the wavefront of a laser typically has a nonuniform spatial power or energy distribution that is stronger in the center than at the outer edges. Furthermore, to make use of the beam, it is often necessary to expand the cross-sectional area of the beam, thereby spreading the non-uniformity across a larger wavefront.
When conventional lenses are used to expand the beam, the non-uniform spatial power or energy distribution of the wavefront is carried through to the expanded beam. In addition, the non-uniformity of the beam becomes more apparent as the wavefront is now expanded over a greater cross-sectional area. This non-uniformity is often detrimental to the performance of a system utilizing the beam as the system must be designed for some average level of beam power or energy or another approach would be to somehow strip the beam of its lesser power outer portions, possibly through the use of an aperture. Neither of these alternatives enable optimum use of the beam's power or energy and it is very difficult to achieve a uniform power or energy distribution, such as the plus or minus one percent variation that is often desired, by way of conventional lens systems.
Holographic elements have been created to function as conventional bulk optical elements. In these cases, the holographic element, whose orientations and spatial periods are correct for the purpose of diffracting the incident wavefront into a desired output location pattern, shape or image. However, when built to function as a basic lens, these holographic elements would also carry the nonuniform spatial power or energy distribution through to the output pattern, shape or image, thereby also inefficiently using the power or energy of the optical source.
The problem of how to compensate for wavefronts having a nonuniform power distribution is addressed U.S. Pat. No. 4,547,037. This patent discloses a multi-faceted holographic element which redistributes the light energy of an incident beam onto a second plane disclosed. This is accomplished by constructing each facet as an individual hologram or diffraction grating. Each facet is sized to be inversely proportional to the intensity of the portion of the beam incident thereupon to assure that substantially the same amount of power passes through each facet. The light transmitted through each facet is diffracted to arrive at different locations on a second plane, relative to their locations in the holographic element. Each of the subholograms or diffraction gratings either expand or contract the portion of the incident beam passing therethrough to illuminate equal, but different, areas on the second plane, thereby producing an output beam at the second plane with a wavefront of nearly constant intensity.
A problem with devices incorporating the teachings of the '037 patent is that if the power distribution of the incident beam upon the surface of the hologram deviates from the design parameters, then the power distribution of the output beam at the second plane will be similarly affected and thus no longer uniform. In optical systems, there are many causes for such deviation in the power distribution of the incident beam could occur. For example, power fluctuations due to the age of the components, or simply the replacement of the source due to failure. In addition, any misalignment within the system due to shock or age will produce an output wavefront having a non-uniform spatial power distribution.
What is needed is an relatively inexpensive way to convert an incident optical beam having a wavefront with a non-uniform spatial power or energy distribution to an output beam having a substantially uniform spatial power or energy distribution that is relatively insensitive to fluctuations in positioning of the incident beam and spatial power or energy distributions within the incident beam. Such a system would function properly with either a continuous wave or pulsed laser as a light source.